Fluorescent lamps play an important role in present-day lighting technology due to their high efficiency, good color rendering and long life. A fluorescent lamp consists of a glass tube with electrodes at both ends, coated on the inside with a phosphor powder. The tube contains a mixture of one or more noble gases (neon, argon, krypton) at a certain pressure and a small amount of mercury vapor. The lamp is operated by maintaining a gas discharge in it, with the help of two electrodes, one at each end of the glass tube. In the discharge, mercury atoms are excited to emit ultraviolet radiation which is transformed to visible light by the phosphor coating on the tube. There are two kinds of lamps, instant strut (cold cathode) lamps and rapid start (hot cathode) lamps. Instant start type lamps have a straight electrode at each end of the lamp, while rapid start lamps have filament coils at each end which need sufficient preheat before the lamp can start without electrode sputtering. The present invention is useful for both types of lamps, with only a slight modification (to provide the filament heating) in the circuit for a rapid start fluorescent lamp. In each case, the ballast is used to stabilize the ac current flow through the lamp.
A problem with fluorescent lamps and other gas discharge lamps is that they cannot be connected directly to a utility power line of 60 Hz (or even 400 Hz on aircraft). They require the addition of a ballast, a typical one of which is shown in the prior-art FIG. 1(a) for a cold cathode (instant start) fluorescent lamp 10a or a ballast for a hot cathode (rapid start) fluorescent lamp 10b shown in FIG. 1(b). The ballast consists of two distinct stages: a first stage 11 comprising an ac-to-dc converter utilizing a conventional full-wave bridge rectifier and a boost current shaper having switches Q1 and D1 for high power factor control, and a second stage 12 comprising a resonant inverter having two switches Q2 and Q3 for output current control and a resonant network to provide the sine-wave current source required by the lamp. The ballast is needed both to start the lamp with high enough voltage as well as to set up the stable operating point, since the fluorescent lamp as a circuit element exhibits a negative impedance once it is energized due to its nonlinear (voltage-current) V-I characteristic.
From the discussion above, it is apparent that the ballast is a two-port (four-terminal) network placed between the utility line and the fluorescent lamp which should have the following functions: supply proper starting and operating voltage; maintain the running current at a design value with a low crest factor; regulate the current output against supply voltage variations; and have a high overall efficiency. A relatively new requirement is to maintain a high (near unity) power factor for the ballast. In addition, since a ballast is a commercial product, low cost, high efficiency and high reliability are also required. Two approaches used in the past to meet these requirements and their relative advantages and disadvantages are:
60 Hz magnetic ballasts: The main advantage of a 60 Hz magnetic ballast is its relative simplicity and, at present, lowest cost. The main disadvantage is its large size and weight requirement due to the low frequency operation, and in particular the heavy magnetics and the associated core and copper losses. Another performance disadvantage for fluorescent lamps is the visible flicker as well as audible noise of the lamp's operation due to the 60 Hz operation. Finally, another major deficiency is its low efficacy, i.e., lower light output for the same electrical input than when the lamp is driven with high frequency currents. The relatively poor power factor of this approach is also a disadvantage.
High frequency electronic ballasts: High frequency (20-60 KHz) electronic ballasts have been widely accepted as an improvement over the 60 Hz magnetic ballasts in that they provide much smaller size and weight, higher efficiency, elimination of flicker and elimination of audible noise. In addition, high (almost unity) power factor, together with higher lamp efficacy, have been achieved due to the operation at higher frequencies. An efficient control via duty ratio modulation in the output controller of the switches Q2 and Q3 is also naturally provided which results in a new performance feature for fluorescent lamps of light dimming (continuously adjustable light output) as needed, resulting in extra energy savings. The major disadvantage of the prior-an high frequency electronic ballast is in its increased cost.
In essence, an electronic ballast for a fluorescent lamp or other gas discharge lamp is a circuit that functions as a high frequency current source. Before the lamp turns on, lamp impedance R.sub.L is infinitely large. This current source characteristic thus generates high enough voltage to start the lamp. After the lamp turns on, lamp current is stabilized at the designed value by the ballast. A resonant inverter can be used as a lamp ballast as shown in FIGS. 1(a) and 1(b) by placing a properly designed LCC band-pass resonant network between the chopped square-wave voltage output of the switches Q2 and Q3 and the fluorescent lamp. A series capacitor Cs of the LCC network is used to block the dc component of the square-wave voltage, while an inductor L2 together with a shunting capacitor Cp and the series capacitor Cs form a series-parallel resonant circuit which provides a current source at its parallel-resonant frequency .omega..sub.Par as described below.
Thus, the LCC network in the resonant inverter 12 can be viewed as shown for the instant start lamp of FIG. 1(a), is driven by a square-wave voltage source with the magnitude V.sub.dc (voltage across the capacitor C1 in FIG. 1(a)) and the pulse width DT.sub.S. Since the LCC network is a band-pass network which is designed to provide a low crest factor sine-wave output current, approximation can be made by considering the fundamental component at the switching frequency .omega..sub.s only. Therefore, the voltage source can be represented by its fundamental component .sqroot.2V.sub.s (D)Sin(.omega..sub.s t) only, where ##EQU1##
The Norton equivalent circuit of the LCC resonant network includes an ideal current source i.sub.o =.sqroot.21.sub.o (.omega..sub.s)Sin(.omega..sub.s t) and a shunting impedance Z.sub.o (.omega..sub.s) seen by the lamp 10(a) in which ##EQU2## where the series branch impedance Z.sub.s (.omega..sub.s) is ##EQU3## and the output impedance is ##EQU4## Therefore, the output impedance Z.sub.o (.omega..sub.s) is ##EQU5## Thus, the lamp sees a current source and the lamp current i.sub.L (rms. value) is therefore equal to the current source I.sub.o (.omega..sub.s).
It can be shown that once the switching frequency .omega..sub.s and lamp current i.sub.L is specified, the LCC network can be determined by ##EQU6##
Note the above results are obtained when all the components are assumed to be ideal, i.e., the non-idealities including saturation voltage of MOSFET switches Q1 and Q2 and the parasitic resistance of reactive components are neglected.
The band-pass characteristic of the LCC network will provide the lamp with the desirable low crest factor sine-wave current. Therefore the LCC resonant network functions as a matching network which transforms the square-wave voltage source at its input to the necessary sine-wave current source to drive the lamp 10a. Note that the above two switches Q2 and Q3 must be current bidirectional since the lamp is an ac load and the LCC network processes the ac current. If the input square-wave voltage does not contain any dc component, then a simple L2, Cp parallel resonant network can be used. The current source characteristic at the parallel resonance shown before is suitable to drive the lamp 10a which will be interpreted here in terms of the Q factor. For the parallel resonant network L2, Cp, Q is defined as: ##EQU7##
Therefore, if lamp current i.sub.L increases, lamp impedance R.sub.L will drop since the ionization of the lamp is increased, then Q in Eq. (7) will also drop due to the decrease of R.sub.L, which in turn will reduce lamp voltage V.sub.L, and finally lamp current i.sub.L is reduced and thus brought back to the designed value and vice versa.
The above resonant inverter is currently one of the most popular ballast topologies due to its minimum component count and inherent current source characteristic at the resonance. Unlike other loads, a fluorescent lamp as a load does not require fast regulation. So the lamp can be driven either open-loop or with slow feedback control. This current source characteristic of a LCC resonant inverter can provide high enough voltage to strike the lamp during ignition and stabilize its running current thereafter.
As noted with reference to FIG. 1(a), an instant start lamp 10a can be directly connected in parallel with the shunting capacitor Cp of the resonant matching network, while for the rapid start lamp 10b shown in FIG. 1(b), sufficient preheat of the filaments may be required before the high voltage is applied to start the lamp to avoid the sputtering of the electrodes which reduces lamp life. The preheat procedure can be provided by either setting a small duty ratio D or operating the ballast at higher frequency, such that the resonant circuit is detuned, and no current source is generated to provide the high voltage during the first couple of seconds. Then parallel resonant frequency .omega..sub.Par or normal duty ratio D can be resumed to start the lamp.
For the rapid start lamps 10b, various filament heating connections including that shown in FIG. 1(b) can be used. In some cases, especially for the dimming ballast, constant voltage filament heating is required and may be separately provided to stabilize the discharge. Therefore the above LCC resonant network shown in FIG. 1(b) can be modified by various filament heating schemes. In addition, multilamp extensions can be easily made by either cascading single lamps or paralleling resonant matching networks in the second stage each with a single lamp in place such that the ballast can be efficiently utilized and the system cost is reduced.
High power factor is now a required feature due to the new international regulations coming into effect which limits the amount of harmonic distortions on the utility line. The benefits of high power factor include reductions in the rms line current and the line current harmonic distortions so that the utility line can be more efficiently utilized and less polluted. The existing high power factor electronic ballast consists of effectively two cascaded power conversion stages as illustrated in FIGS. 1(a) and 1(b). The first power conversion stage is designed to provide a high power factor ac-to-dc rectification from the utility line. The second switching power stage is a dc-to-ac converter controlled to provide the necessary high frequency ballasting function Both passive and active high power factor ac-to-dc converters can be used as the first stage to shape the input current, but in each case the ballast consists of two stages in cascade which makes the prior-art ballasts large in size and costly with much less efficiency and reliability than could be achieved with a single-stage ballast having all of the stone desirable characteristics plus a high input power factor approaching unity. It is this plus feature that requires the two-stage ballast configuration of the prior art.
Passive current shapers have the advantage of simple structure but suffer from the size and weight of the line frequency reactive components. In addition, power factor increase or harmonic distortion decrease is not satisfactory. In some cases, the line frequency modulation on the high frequency lamp current can not be eliminated. An active current shaper, such as the boost current shaper 11 shown in FIG. 1(a), has the advantage of having a unity power factor and a compact size by processing the input power at the switching frequency, but with the high power factor controller the circuit is complicated and thus expensive. On the other hand, the resonant inverter with the properly designed resonant matching network can be used as the second stage in FIG. 1(a). The inverter 12 comprises a pair of current bidirectional switches and an output controller 14 to generate square-wave voltage at the switching frequency and a matching network comprising reactive (resonant) components (such as in an LCC network) to generate the sine-wave current source at the output. For that reason, the resonant inverter comprising two current bidirectional switches and the resonant matching network is a favorite candidate as the second stage of a ballast to drive a fluorescent lamp due to its low component count and simplicity of the output controller for open-loop or closed-loop operation.
The major drawback of all prior-art, high power factor electronic ballasts is that they consist of two cascaded power conversion stages as just described. Clearly, this approach has a number of deficiencies, all of them stemming from the use of the two cascaded power conversion stages:
1. Reduced efficiency due to processing of the input power twice through two stages; PA1 2. Increased size and weight; PA1 3. Doubling the cost and reduced reliability due to the two power processing stages.